SD card reader for ZX81

[siggi]

There are some tests pending for an issue with HR graphics, which i hope to finish in the next few days.

Hi Sascha
what is the ussue with HR graphics? Are some pixels flickering?
If yes, that is a timing problem Wilf Writer wrote about: the /MREQ-signal during refresh (while hrg pixels are read from ram) is much shorter than usual. Then the access time to ram is too short.
The solution is to feed (/MREQ AND /RFSH) to the memory address decoder instead of only /MREQ to get the "normal" timing during refresh.

Regards
Siggi

[sittner]

Hi Siggi,

I think I've sorted out things a little bit:

First I've tried to disable ZX81's internal memory completely and only use the expansion board's memory to simplify things. Then the ZX81 came up with this pic after switched on:

After some investigations and a look to the ZX81 schematics I've remembered why I've done the trick with the diode between /RFSH and /ROMCS_INT (which is the chip select of the internal ZX81 ROM): The addresses for the font lookup (while /RFSH is low) will only reach the internal ROM, since it's address bus is loosely connected to the CPU bus via R18-R26. So the ULA is able to apply the fake addresses (only) to the internal ROM.

Since I was not very happy with the diode stuff and the potential risk of activating two data bus line drivers at the same time I've tried to generate all the needed signals, which allows to enable the internal ROM at the critical points:

Now the text mode is working again and even the TESTBILD.P from Bodo Wenzel is working (which already was with the original logic):

The open question is: why is HRDEMO3.P not working (should it)?

I'm not 100% sure about the required memory layout. Currently it looks like that:

0x0000 - 0x3FFF: lower 16K of ROM
0x4000 - 0x7FFF: lower 16K of RAM
0x8000 - 0xBFFF: upper 16K of ROM
0xc000 - 0xFFFF: upper 16K of RAM

Do you have any idea or suggestion?

Thank You & Kind Regards

Sascha

[siggi]

Is it Wilf Rigter's hires demo?
Wilf's "WRX" hires stores its hires image usually at address $2000 in ram (Wilf made and sold a static 8K ram card for that) .

About /ROMCS-int (for the interal rom): never drive this line directly by an ic. You should only disable it (by using a diode to drive the line to HIGH only) and leave it as is, if you do not need its address range. The reason is, that other cards connected to the edge connector might also need to disable the internal rom and will drive /ROMCS high.

Siggi

[sittner]

Wilf's "WRX" hires stores its hires image usually at address $2000 in ram (Wilf made and sold a static 8K ram card for that) .

I've tried to find a way to enable RAM in this area, but I've found no (simple) way. So I will give up WRX16 for now. I've added some notes about this in the README.md.

About /ROMCS-int (for the interal rom): never drive this line directly by an ic. You should only disable it (by using a diode to drive the line to HIGH only) and leave it as is, if you do not need its address range. The reason is, that other cards connected to the edge connector might also need to disable the internal rom and will drive /ROMCS high.

That's a good point. I have added a diode to take this into account.

Thank you much for your support, that helped a lot.

Kind Regards
Sascha